This invention relates to an optical sensor for detecting chemical species. More particularly, the invention relates to an optical sensor that relies upon interference enhanced reflection (hereunder abbreviated as IER) for detecting chemical species using reflected light from a thin organic or high-polymer film that functions as a sensing element. The thin film either reacts with or adsorbs gaseous or liquid chemical species.
Various types of optical sensors are known that rely upon the reflection of light for operation. One of the most recent examples is the proposal by Gauglitz et al., who reported a method of reflection spectroscopy for detecting gases or solvents using the swelling of polymer films (GIT Fachz. Lab., 7, 889, 1990). This method uses a white light source and a spectrometer for analyzing the spectral reflectance; hence, the equipment for implementing this method is bulky with the additional need to use an optical filter and other peripheral parts.
European Patent Publication No. 0 282 009 A2 teaches a fiber optical detector that makes use of the change in reflectance due to the interaction with hydrocarbons. The operating principle of the detector described in the publication is that the refractive index of the cladding will change in the presence of hydrocarbons thus causing changes in optical losses.
Nylander et al. discussed a gas sensor that relied upon surface plasmon resonance for operation (Sensors and Actuators, 3, 79-88, 1982). The conditions for resonance of surface plasmons that occur in the sensor are largely dependent on the optical parameters of the polymer film used in the sensor, which in turn will vary upon exposure to the vapors of organic matters. U.S. patent application Ser. No. 259,556 (Oct. 18, 1988) teaches a fiber optic sensor for chemical species that has a thin metal film formed at an end of the optical fiber. The operating principle of this sensor is as follows: when the thin metal film is brought into contact with a chemical species, the thickness of the film and, hence, its reflection characteristics will vary.
Butler reported an interference sensor that had a polymer film formed at an end of a multimode optical fiber. The operating principle of this sensor is that when the polymer is brought into contact with a chemical species, it will swell to thereby change the reflecting property of the fiber and at which the polymer is formed. A problem with this sensor is that it is not easy to form a thin polymer layer at an end of the optical fiber.
Japanese Laid-Open Patent Application No. 77651/1992 describes a fiber optic sensor of chemical species that uses a sensing membrane containing both a dye and a chemical species sensitive compound. The operating principle of this sensor is that when the chemical species sensitive compound binds with the chemical species to be detected, the sensing membrane will experience a change in light reflectance or color.
The fiber optic sensor described above have one problem in common; that is, the change in reflectance that can be observed is so small that no satisfactory sensitivity is attainable, whereby the dynamic range of the sensors is limited.
Japanese Laid-Open Patent Application No. 19042/1922, 133900/1993 and 142152/1994 disclose a chemical sensor for ammonia and amine which uses a conductive polymer thin film on a transparent conductive substrate. The reflectance of the film changes when it is contacted with a chemical substance to be detected. However, the sensor is based on a simple reflection measurement that does not utilize the optical interference techniques. The present invention uses the IER method that specifically relies on optical interference from a thin structure. U.S. Pat. No. 4,641,524 teaches an optical humidity sensor that is equipped with a mirror having a layer of a porous water-absorbing dielectric material. U.S. Pat. No. 4,525,376 teaches a thickness regulator that relies upon wavelength-dependent reflection for monitoring a silicon layer on a substrate.
Japanese Laid-Open Patent Application No. 19142/1992 describes an ammonia sensor which utilizes a change in light reflectance on the surface of a polymerized film having an electrically-conductive polymer. However, this sensor does not exhibit quick response and thus a sensor with quicker response has been demanded.
An object of the present invention is to provide a sensor that has a higher sensitivity and selectivity, that features quick response, and that is small enough to be constructed easily.
According to the present invention, there is provided an optical sensor for detecting gaseous or liquid chemical species that includes a sensing element having a thin film and a means of measurement by IER.
The present invention also provides an optical sensor that uses a thin organic or inorganic film or a thin high-polymer film as the thin film in the sensing element.
The present invention further provides an optical sensor that is capable of detecting a single or a plurality of chemical species as distinguished from each other by measuring the change in the relative intensity of reflected light or the response on the basis of the swelling of the thin film or the changes in its optical parameters.
The present invention also provides an optical sensor that comprises a series of sensing elements which are made of similar or dissimilar materials for achieving simultaneous detection of different chemical species, increasing the precision of identification, and correcting background effect.
The present invention further provides an optical sensor that accomplishes not only quick response but also high sensitivity and broad dynamic range by selecting proper substrate materials, optimal values for the thickness of the thin film in the sensing element and the angle of incidence of light.
The present invention also provides an optical sensor incorporated in a detection system that comprises a light source, a polarizer, optical paths for reference light and signal light, photodetectors, and electronic circuitry.